1Introduction

The first and second Strategic Arms Reductions Treaties (START I and START II) call for deep reductions in the strategic nuclear forces that have been deployed on the territories of the United States, Russia, and three of the other states of the former Soviet Union. The United States and Russia have decided to undertake even deeper reductions in tactical nuclear weapons under reciprocal unilateral initiatives. If these agreements and initiatives are fully implemented, the United States and Russia will have large numbers of nuclear warheads that are not required either for deployment or for reserves and stockpile support. We call these "excess" warheads. Some 10,000-20,000 warheads in the United States and at least a similar number in the former Soviet Union are likely to fall into this category, depending on the ultimate scope of reductions and decisions concerning the size of nondeployed reserves. These excess nuclear weapons on the two sides could contain well over 100 metric tons of plutonium, and perhaps 1,000 metric tons of highly enriched uranium (HEU), much of which may also be declared excess to military needs.1

Given this situation, the United States has an interest in:

minimizing the risk that either weapons or fissile materials could be obtained by unauthorized parties;

1

Throughout this report we use metric tons (MT) as the measure of the amounts of plutonium and HEU; all references to tons are to metric tons. One metric ton is 2,205 pounds, roughly 10 percent larger than an English ton.

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
1
Introduction
The first and second Strategic Arms Reductions Treaties (START I and START II) call for deep reductions in the strategic nuclear forces that have been deployed on the territories of the United States, Russia, and three of the other states of the former Soviet Union. The United States and Russia have decided to undertake even deeper reductions in tactical nuclear weapons under reciprocal unilateral initiatives. If these agreements and initiatives are fully implemented, the United States and Russia will have large numbers of nuclear warheads that are not required either for deployment or for reserves and stockpile support. We call these "excess" warheads. Some 10,000-20,000 warheads in the United States and at least a similar number in the former Soviet Union are likely to fall into this category, depending on the ultimate scope of reductions and decisions concerning the size of nondeployed reserves. These excess nuclear weapons on the two sides could contain well over 100 metric tons of plutonium, and perhaps 1,000 metric tons of highly enriched uranium (HEU), much of which may also be declared excess to military needs.1
Given this situation, the United States has an interest in:
minimizing the risk that either weapons or fissile materials could be obtained by unauthorized parties;
1
Throughout this report we use metric tons (MT) as the measure of the amounts of plutonium and HEU; all references to tons are to metric tons. One metric ton is 2,205 pounds, roughly 10 percent larger than an English ton.

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
minimizing the risk that weapons or fissile materials could be reintroduced into the arsenals from which they came, halting or reversing the arms reduction process; and
strengthening the national and international arms control mechanisms and incentives designed to assure continued arms reductions and prevent the spread of nuclear weapons.
In pursuit of these objectives, the U.S. and Russian governments have agreed that the United States will buy 500 tons of excess Russian HEU, which will be "blended down" to low-enriched uranium (LEU)2 so that it can be used for nuclear reactors but not for weapons. The United States will later resell the material to fulfill demand for nuclear fuel on the domestic and world market.
Plutonium, though it can be used as a fuel in either current or future reactor designs, does not lend itself to the commercial approach being taken with HEU for two reasons: (1) plutonium cannot be isotopically diluted so that it cannot be used in weapons; and (2) it costs more to use as fuel in current light-water reactors than LEU, even if the plutonium is available free while the uranium must be mined, processed, and enriched. Plutonium's toxicity and the need to safeguard it from diversion and theft3 require special handling procedures that greatly increase the costs of its use.
Accordingly, in 1992 the U.S. government asked the Committee on International Security and Arms Control (CISAC) of the National Academy of Sciences (NAS) to identify and evaluate the approaches that could be used for the disposition of plutonium from excess nuclear weapons. The Panel on Reactor-Related Options for the Disposition of Excess Weapons Plutonium was formed by the NAS in November 1992 to support CISAC's study. The panel consists of three members of CISAC (Richard Garwin, John Holdren, and Michael May) and four additional members selected for their relevant expertise on issues related to reactors and reactor wastes (John Ahearne, Robert Budnitz, Thomas Pigford, and John Taylor) (see list of panel members on p. iii).
2
Natural uranium includes only 0.7 percent of the fissile isotope uranium-235 (U-235), with almost all of the remaining 99.3 percent being U-238. (Isotopes are different forms of the same chemical element having differing numbers of neutrons—235 neutrons and protons together in the case of U-235, and 238 in the case of U-238.) To be usable in a weapon, the concentration of U-235 must be greatly increased from its level in natural uranium, a process known as enrichment. Typical weapons-grade uranium contains more than 90 percent U-235. Because the various isotopes of an element are chemically indistinguishable, enrichment requires physical separation techniques. These are costly and time-consuming, and the relevant technology is not widely available—a set of circumstances that has constituted one of the primary technical barriers to proliferation of nuclear weapons. Some types of nuclear reactors, by contrast, can operate with natural uranium, though most of the world's power reactors use LEU containing 3-5 percent U-235. HEU can easily be blended with natural or depleted uranium to produce LEU.
3
We follow the Committee on International Security and Arms Control report (NAS 1994, p. 6) in using the term "diversion" for cases in which the state that owns the material returns it to weapon use and the term "theft" for cases in which the material is illicitly obtained by other parties.

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
Analyses prepared by the panel served as input to the deliberations of CISAC in its broader charge, which included consideration of disposition options not related to nuclear reactors, as well as issues of preliminary storage and management of the weapons plutonium (WPu). CISAC's report was released in early 1994 (NAS 1994). Completion of the current report of the Reactor Panel, which in some respects is more detailed than the analyses originally presented to the parent committee but does not alter in any significant way the conclusions of those analyses, required additional time. The responsibility for the content of the panel's report, which has been subjected separately to the Academy's review process, rests solely with the members of the panel; similarly, the non-CISAC members of the panel bear no responsibility for the conclusions that CISAC draws, in its own report, from this and other inputs.
We took the task of our panel to be the identification and illumination of all those options for the destruction or alteration of WPu that entail either emplacing and irradiating it in nuclear reactors or immobilizing it in waste forms similar to those contemplated for disposal of fission-product wastes originating in nuclear reactors. (CISAC decided to include the latter options within our purview because of the familiarity of some of our panel members with radioactive waste vitrification technologies and the related radioactive waste management issues.)
The reactor-related options for disposition of excess WPu can be divided into several categories. Some options (described as "minimized accessibility" options in the CISAC report) would destroy only a fraction of the plutonium, but would increase the difficulty of recovering it and using it in nuclear explosives. Reactors, for example, could accomplish this objective by using the plutonium in a once-through cycle, leaving a significant amount of plutonium (different in its isotopic characteristics from the original plutonium) embedded in spent fuel that would also contain intensely radioactive fission products. We call this category of options the "spent fuel" approach. Alternatively, the plutonium could be irradiated in reactors more briefly (accomplishing the job faster, at the cost of providing a smaller barrier to weapons use of the material); this we refer to as "reactor spiking." Similar objectives could be accomplished by immobilizing the plutonium in a solid waste form—such as a glass, a ceramic, or a cement—which might or might not include radioactive fission products from prior reprocessing of nuclear fuel; we call this the "immobilization” approach. Under this approach, the isotopic composition of the plutonium would remain weapons-grade, a factor whose significance we examine in later sections.
Finally, it is also possible in principle, through repeated separation and reuse of the plutonium in the spent fuel, to consume the plutonium nearly completely, so that the resulting wastes would no longer require significant safeguards. Compared to the other approaches, this "elimination" approach could not be begun as quickly and would require a much longer time to complete, as

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
well as entailing substantially higher costs and technical, political, and institutional uncertainties.
Within each of these approaches, there is a variety of possible technical options that might be used (involving different reactor types, waste treatment approaches, and the like). In this report, we identify these options, and for each one we offer a description of its general characteristics, its state of technological development (including the steps remaining to be accomplished before it could be made operational), and the characteristics of the option as a means of WPu disposition (including the rate at which the plutonium could be processed, the characteristics of the final form in which the plutonium would be left, and the various security risks involved in reaching that final form). We then assess and compare the options on the basis of a variety of criteria: timing and other aspects of security; cost; and environment, safety, and health. Our recommendations rely heavily on these comparisons.
ROAD MAP OF THE REPORT
This report is organized as follows. In the remainder of the introduction, we outline some of the assumptions related to security issues and goals of disposition, as well as the broader context of the issue, that we have taken from the parent study. Chapter 2 provides background information on a variety of topics: principles of nuclear physics and engineering relevant to the plutonium disposition task; the characteristics of weapons-grade and reactor-grade plutonium; a general scheme for classifying the various approaches to plutonium disposition within our purview; the amount of plutonium likely to be excess to military needs in the United States and Russia, in the context of the overall world stocks of plutonium and HEU; and the character, number, and scale of nuclear facilities around the world that may be relevant to the plutonium disposition mission. Chapter 3 introduces and elaborates the criteria we think relevant to comparative assessment of reactor-related options for plutonium disposition. Chapter 4 examines the options involving irradiating the WPu in nuclear reactors (including accelerator-driven subcritical reactors). Chapter 5 examines the immobilization options. Chapter 6 provides analysis and conclusions concerning the relative merits of the various options when judged against the criteria presented in Chapter 3. Finally, Chapter 7 summarizes our conclusions and recommendations.
UNCERTAINTIES
It is inevitable that sizable uncertainties would burden any study of the topic we were assigned, even in the absence of the time constraints that affected our particular effort. Some of the options we consider involve reactors, fuel types, or reprocessing schemes that have never been tested at all, or that have

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
been tested only at scales much smaller or for times much shorter than those relevant to the plutonium disposition mission. In some cases, important questions relating to performance, safety, or economics remain to be resolved. In the past, nuclear industry projections about time and cost needed to develop new options, and about the costs of using these options once they are developed, usually have proven to be underestimates. The time lags before particular options are deployable, moreover, will depend on corporate, regulatory, political, and public decision processes whose outcomes cannot be predicted with confidence. The bilateral, and possibly multilateral, agreements that may be involved in carrying out plutonium disposition are also a cause of uncertainty, particularly with respect to timing.
All we can do in dealing with such uncertainties is to characterize their origins, magnitudes, and implications for our findings as forthrightly and clearly as possible. When the uncertainties are such as to call into question the very availability of an option, or its availability in a time frame of interest for our purposes, we call particular attention to them. We do so also where there is reason to believe that further focused research effort could reduce important uncertainties relatively quickly.
GOALS, TIMING, AND RELATED FACTORS
As already noted, the primary goal of long-term disposition of excess WPu should be to reduce the risks to national and international security associated with this material's existence. These risks arise in a complex international context that includes global efforts to reduce nuclear arms and prevent their spread, ongoing unrest in the former Soviet Union, and expanding civilian use of plutonium as a nuclear fuel in several countries. These contexts are described in CISAC's 1994 report, and we will not repeat that description here. Rather, our task is to investigate the extent to which reactor-related options could be used to reduce these security risks while meeting the other criteria described in Chapter 3. We address these contexts only as they relate directly to the security risks posed by the various disposition options.
The Importance of Timing
One important question with respect to long-term disposition of excess WPu needs to be addressed at the outset: Why bother? Why not simply continue to store this material indefinitely?
There is no doubt that interim storage lasting for a decade or decades will be necessary because of the time required to implement any of the plausible long-term disposition options. Extending such storage indefinitely would not be an acceptable approach, however, for two reasons. First, maintaining this material in readily weapon-usable form for an indefinite time, with no chosen plan

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
for its disposition, would send negative political signals for nonproliferation and arms reduction, suggesting that the current arms reductions could be rapidly reversed at any time. Second, indefinite storage of such readily usable nuclear-weapon material would extend substantial risks of diversion and theft into the indefinite future. For these reasons, we believe that it is urgent to begin now to work toward consensus on appropriate disposition options and to implement these options expeditiously once a decision is reached. Indeed, we believe that timing—the time when particular disposition options could realistically be expected to begin operations (considering technical, political, economic, and institutional factors), and how much time would be needed to complete those operations—is one of the key security criteria by which disposition options should be judged, as it will determine how rapidly the liabilities of prolonged storage can be diminished.
In addition to these security issues bearing on timing, some social concerns are also relevant. The communities where plutonium is now stored, or where storage facilities may be constructed, are demanding assurances that storage will have an end-point—that these communities will not become, in effect, permanent plutonium dumps. Such assurances cannot be credibly provided until a plausible road map for ultimate disposition of the plutonium is agreed. Similarly, those with potential commercial or other interests in plutonium disposition, such as reactor vendors or the personnel at government sites that might be involved, deserve timely answers as to whether their capabilities are likely to be employed in this endeavor.
In most cases, the monetary costs of storage are not a major factor contributing to the urgency of disposition. In the United States, the surplus WPu is currently being stored in the form of intact "pits" 4—avoiding the costs of processing this material into some other form—in simple but highly secure bunkers (called "igloos") at the dismantlement site. Since all the security required for handling nuclear weapons would be needed at this site in any case, the net additional cost of storing plutonium there is modest. While the United States is considering construction of a more advanced and costly storage facility, decisions about the further disposition of nuclear-explosive materials resulting from disarmament might not have a decisive impact on decisions concerning the need for such a facility, as this site is designed to store not only weapon components resulting from dismantlement but also plutonium in a variety of residue and scrap forms currently stored throughout the U.S. weapons complex. Russia is planning to build a new storage site with U.S. assistance, but once this facility is built (assuming that it will be), the annual cost of continued storage will again be modest, having little impact on decision-making concerning long-term disposition. This situation stands in contrast to that which obtains in the commer-
4
"Pit" is the term for the fission-explosive core of an implosion-type nuclear weapon. See also the discussion in "Some Relevant Aspects of Weapon Science" in Chapter 2.

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cial plutonium market, where reprocessors charge roughly $2-$3/gram-year for storage of civilian separated plutonium—a charge that would amount to $200-$300 million per year if applied to 100 tons of WPu.
Cost and Other Objectives
Having noted that timing and other aspects of security are crucial to sensible choices about plutonium disposition, we also point out at the outset of our analysis some factors that should not be allowed to drive decision-making regarding plutonium disposition.
For example, although cost inevitably will be a consideration in choosing among long-term disposition options, this factor is of distinctly secondary importance. The net discounted costs of the most promising options we identify amount to between hundreds of millions and a few billion dollars spread over a period of more than a decade—quite small on the scale of ongoing military expenditures designed to reduce major security risks. Differences in the costs of options within this range deserve far less weight in decision-making than differences in security.
Similarly, exploiting the energy value of this plutonium should not be a central criterion for choosing among disposition options. While the stockpile of excess WPu looms large in security terms, amounting to the equivalent of many thousands of nuclear weapons, it is quite small in energy terms—less than a tenth of the plutonium in the world, and the equivalent of only a few months' fuel for the world's power reactors. As described in detail later in the report, moreover, it would be costlier to fabricate free plutonium from weapons into reactor fuel for today's most widely used and economical reactor types than to purchase low-enriched uranium fuel for these reactors, and under plausible assumptions about uranium availability and associated processing costs over the next few decades, the economic disadvantage of plutonium fuel is unlikely to disappear any time soon. Whatever economic value this plutonium might someday represent is small by comparison to the security stakes. Thus plutonium disposition is fundamentally a problem of security, not energy. The cost of management and disposition of WPu must be seen as an investment in security, just as the cost of its production was once viewed.
For much the same reasons, the need for disposition of this small fraction of the world's plutonium stock should not drive decisions regarding the future of civilian nuclear power. That future depends on economic, political, and technical factors outside the scope of this study. We do not take a position on what type of nuclear-energy future would be most appropriate for the United States, the countries of the former Soviet Union, or the world. The stock of excess WPu is not essential to the future of any civilian nuclear development programs. Of course, our consideration of reactor options for the disposition of WPu must take account of the capabilities now available and likely to become available in

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Management and Disposition of Excess Weapons Plutonium: Reactor-Related Options
the next decade or two in the civilian nuclear-energy-generation sector, and our findings may include insights about the relative diversion resistance of reactor and fuel-cycle combinations that are candidates for civilian use. But national decisions about the future of nuclear energy will (and should) be shaped by many factors in addition to plutonium disposition and plutonium safeguards concerns. We view our task as illuminating the plutonium disposition possibilities that different reactor options would offer, not as recommending national nuclear-energy strategies on the basis of these possibilities.
Finally, while tritium production issues were not part of the panel's charge, and we have not examined tritium production in detail, we see no essential reason why plutonium disposition and tritium production need be linked. The arguments on this point are outlined in CISAC's report (NAS 1994, pp. 152-153).
U.S. AND RUSSIAN PLUTONIUM DISPOSITION: DIFFERENCES AND LINKAGES
One critical part of the context for disposition of WPu is the ongoing political, economic, and social turmoil in the former Soviet Union. These crises have raised doubts about whether more than one nuclear-armed state will arise from the former Soviet Union, whether recent arms reduction agreements and pledges will be successfully implemented, and whether adequate accounting and security for nuclear weapons and fissile materials is being maintained. Recent seizures of smuggled weapon-usable materials highlight the urgency of these concerns. In the case of plutonium disposition, these crises raise questions concerning whether the economic resources and political attention needed to ensure secure management and disposition of fissile materials will be available in the near term.
The optimal approaches to long-term plutonium disposition may be different in the United States and Russia.5 The risks involved in storing, handling, processing, and transporting plutonium are much higher in Russia under present circumstances, and the two countries' economies and plutonium fuel policies are very different. The most important officials and agencies in the Russian government strongly prefer disposition options that use surplus WPu to generate electricity in reactors. To convince Russia to pursue, in the near term, options that dispose of the plutonium without deriving electricity from it would be difficult (though perhaps not impossible, particularly with sufficient financial incentives).
While U.S. and Russian disposition approaches may differ, rough parallelism in the timing and scale of long-term disposition would be desirable, so that
5
If current arms agreements are successfully implemented. all the nuclear weapons of the former Soviet Union will be transported to Russia for dismantlement, leaving Russia alone with the burden of dealing with the excess plutonium from arms reductions.

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both nations' available plutonium stocks would remain comparable. After long-term disposition, neither nation's excess plutonium should be greatly more accessible for use in weapons than the other's.
U.S. consideration of the risks associated with the various options for management and disposition of its own plutonium must be informed by an awareness of the potential linkages between U.S. choices and the choices that may be made in the former Soviet Union. U.S. policy could affect the management and disposition of excess WPu in the former Soviet Union in a variety of ways—including financial assistance, negotiated agreements to pursue particular approaches, outright purchase of former Soviet WPu, or merely setting an example. What is done with excess WPu in the United States and the former Soviet Union, moreover, could affect the fate of the substantially larger (and still growing) quantities of separated and unseparated plutonium discharged from civilian nuclear power reactors worldwide. This study, therefore, will consider disposition of both U.S. and Russian excess WPu, although necessarily with more detailed attention to the U.S. case.
REFERENCES
NAS 1994: National Academy of Sciences, Committee on International Security and Arms Control. Management and Disposition of Excess Weapons Plutonium. Washington, D.C.: National Academy Press, 1994.